1. Field of the Invention
The present invention relates to an exhaust gas cleaning device constructed of a metal-made honeycomb core body for carrying and exhaust gas cleaning catalyst. As a cleaning means for exhaust gas from a motor vehicle, the exhaust gas cleaning deivce is generally installed at an intermediate point of an exhaust pipe.
More specifically, this invention is concerned with an exhaust gas cleaning device of the above sort, which is employed under severe conditions. The exhaust gas cleaning device features the use of a honeycomb core body with improved resistance to deformations and breakage by thermal expansion and stresses.
2. Description of the Related Art
Conventionally, an exhaust gas cleaning device of the above sort is constructed of a cylindrical metal casing opening in both ends thereof and a multi-layered honeycomb composite body (hereinafter called the "honeycomb core body") inserted and fixed in the metal casing. The honeycomb core body is generally fabricated by superposing a planar band made of a heat-resistant steel sheet and a corrugated band obtained by forming a similar steel sheet into a wavy or sinuous form one over the other in a contiguous relationship and then rolling the thus-superposed bands together into a multi-layered spiral form.
The honeycomb core body and metal casing are firmly joined together by brazing or soldering, so that they can withstand thermal stresses and deformations due to the high temperature of exhaust gas, exothermic reactions between exhaust gas and the catalyst and the like and further vibrations while the associated motor vehicle is running. Needless to say, the planar and corrugated bands which make up the hooneycomb core body are firmly joined together at contacts therebetween by any one of various suitable methods, for example, by welding or brazing.
There is however a recent move toward constructing an exhaust gas cleaning device without a separately-fabricated metal casing, in other words, with a metal-made honeycomb core body alone from the stand point of the price competition with conventional cordierite ceramic carriers. The omission of such a metal casing naturally leads to a reduction in the cost for inserting a metal-made honeycomb core body in the metal casing and then uniting them together, namely, in the so-called canning cost, whereby a substantial cost merit can be brought about.
However, the conventional exhaust gas cleaning devices which are each constructed of a honeycomb core body of the above-mentioned sort only or of such a honeycomb core body and a metal casing of the above-described sort cannot withstand long-term application. Especially, along the longitudinal axis of each honeycomb core, in other words, in the direction of flow of exhaust gas, deformation forces nad strains which are caused by thermal expansion and thermal stresses occurred in members (i.e., planar band and corrugated band) making up the honeycomb core body are concentrated on an area adjacent the outer peripheral wall of the honeycomb core body or an area in the vicinity of the outer peripheral wall of the honeycomb body and the inner peripheral wall of the metal casing. This tends to lead to the problems that the planar and/or corrugated bands of the honeycomb core body are subjected to considerable separation, cracking and/or breakage at the area adjacent the outer peripheral wall of the honeycomb core body and the honeycomb core body and metal casing are separated.
This has led to the need for a change to the belief that firm connection should preferably be formed between the planar band and the corrugated band, which make up the honeycomb core body, or between the outer peripheral wall of the honeycomb core body and the inner peripheral wall of the associated metal casing.